The measurement of the porosities of subsurface earth formations surrounding a well borehole by means of the attenuation of neutron flux with distance from the neutron source is well known in wireline logging. Epithermal logging tools, in particular, are sensitive to the hydrogen density or concentration in a formation. As hydrogen is generally found in formation fluids, hydrogen concentration is related to the amount of pore space, and thus the porosity, of the formation. For a given porosity, however, an increase in matrix density (keeping the same matrix composition) can cause an epithermal neutron detector count rate (for a source-to-detector spacing of 60 cm for example) to decrease. This change in count rate is the same direction as would occur if the porosity increased for a given matrix density. Thus a neutron porosity measurement by itself cannot unambiguously determine the porosity of a formation of unknown composition.
It is conventional in wireline logging, therefore, to make bulk density measurements of a formation of interest by running a second tool, based on Compton scattering of gamma rays from electrons, over the same depth interval as the neutron porosity tool. An increase in matrix density also causes a decrease in the detector count rate in the density tool. On the other hand, if the porosity increases for a given matrix density, the density tool detector count rate increases. Changes in matrix density and porosity thus have complimentary effects on neutron porosity and Compton-scattering density tools, which effects can be offset by cross plotting the responses of the two tools. By use of such cross plots, the physics can be untangled and changes in matrix density and composition (lithology) can be determined. Because the inclusion of gas in the matrix pore spaces also affects the neutron porosity and density tool responses, it is possible in certain circumstances to detect the presence of gas by means of neutron/density cross plots.
The conventional bulk density measurement technique, however, requires a source of gamma rays, typically a .sup.137 CS isotopic source. Such radioactive chemical sources have obvious disadvantages from a radiation safety viewpoint. This is of particular concern in measurement-while-drilling applications, where operating conditions make both the loss of a source more likely and its retrieval more difficult than in wireline operations. Indeed, the aforementioned measurement-while-drilling prior art patents have focused in substantial part on preventing the loss or, if lost, the recovery of such chemical sources.
Although accelerator-based wireline porosity tools are known, see, for example, U.S. Pat. No. 4,760,252 to Albats et al., there currently is no practical and economical accelerator-based alternative to the .sup.137 Cs gamma ray source for density logging. A need exists, therefore, for an accelerator-based tool which would eliminate the requirement for the radioactive chemical sources of conventional bulk density tools.